WHAT SIZE DO YOU NEED Large (this item) = palm width of 8.9cm or more; Small/Medium - palm width of 8.8cm or less. To activate the scroll function on the 3M Ergonomic Mouse you have 2 simple-to-use options. You can either press down and hold the middle part of the black button on top of the Mouse to scroll up and down or alternatively, you can press the black rectangular button on the neck of the Mouse. This then brings up a circular symbol containing 2 black arrows - one pointing upwards and the other downwards. Just move your mouse in the same direction as the arrow to move quickly up and down the page. Please note you do not need to hold down the rectangular button in order to scroll up and down. When you have finished with the scroll function just click the black rectangular button a second time to exit this function.